Principles of Building Envelope Performance...2014/01/02  · Source: Graphic adapted from Wood...

Learning Objectives

1. The purpose of the building envelope

2. The energy code’s definition of the building envelope

3. The exterior forces that act on the building envelope

4. The interior forces that act on the building envelope

5. The concept of continuous control layers to resist

exterior and interior forces

2

The Building Envelope Defined

The building envelope is

the part of a building that

separates the controlled indoor

environment

from the uncontrolled outdoor

environment

3

The Building Envelope Defined

2009 International Energy Conservation

Code (IECC):

Building Thermal Envelope: The basement

walls, exterior walls, floor, roof, and any other

building element that enclose conditioned

space. This boundary also includes the

boundary between conditioned space and any

exempt or unconditioned space.

4

The Building Envelope Defined

[2011 NYCECC]

Building Thermal Envelope: The basement walls,

exterior walls, floor, roof, and any other building

element that encloses conditioned space. This

also includes the boundary between conditioned

space and any exempt or unconditioned space

http://www.nyc.gov/html/dob/html/codes_and_reference_materials/nycecc_key_definitions.s

html

Building Envelope Design

Factors

Control heat flow

Control vapor diffusion

Be economical Be easy to build

Control radiation and light

Provide security

Control sound transmission

Control fire

Resist imposed loads

Control rain penetration

Control air flow

Accommodate movement

Be aesthetically pleasing Be durable

7

Environmental Factors

Load: vapor pressure Load: air pressure

Load: temperature

Exterior Environment Temperature

Air Movement

Humidity

Rain

Snow

Light

Seismic

Soil Environment Temperature

Air Movement (radon gas)

Humidity

Seismic

Interior Environment Temperature

Air Movement

Humidity

Light

Occupant

activities

Mechanical

design

Air leakage

Snow cover

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Wind

Durability

• Ability to continue to perform functions over time

• Durability is a function of a material and its

environment

• E.g., heat, UV radiation, thermal cycles, wind, salt

spray, movement, water, biological growth, etc.

degrade building materials

• Moisture is the single biggest environmental factor

affecting durability

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Hydrothermal Design Factors Heat, air, and moisture control

Control

Heat

Flow

Control

Rain

Penetration

Control

Air Flow

Control

Vapor

Diffusion

Heat, air and

moisture flows are

interrelated

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Heat, air and moisture flows

are Interrelated

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Control

Air Flow

Control

Heat

Flow

Control

Vapor

Flow Control

Rain

Penetration

Water in the Environment

• “Water” exists in three phases:

• Liquid (rainwater, groundwater)

• Solid (snow and ice)

• Gas (water vapor)

• Building envelope

design must account

for water in all three

phases.

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Relevant terms related to

vapor

• Relative Humidity: defined as the ratio of partial

pressure of water vapor to the saturated vapor

pressure.

• Dew point Temperature: the temperature at which

air containing a constant amount of water vapor

reaches the saturation point. As the temperature

decreases, the air has a lower capacity to contain

moisture. Condensation can occur at or below the dew

point temperature.

• Condensation: Change of state of water from gas to

liquid, due to the water vapor cooling and contacting a

surface at or below the dew point.

13

Water Vapor Diffusion

• Vapor diffusion is the process of water vapor

molecules moving through a material

(independent of air movements)

Source: Graphic adapted from Wood Frame Envelope in the Coastal Climate of British Columbia Best Practice Guide,

Canada Mortgage and Housing Corporation.

Inside Air

high water

content

Outside Air

low water

content

Direction of vapor flow

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Air Leakage

Air leakage is driven by the

air pressure difference across

the building envelope assembly.

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Air infiltration occurs

where “smoke” is

sucked into building

“Smoke” puffer

Air Pressure Difference

Stack Effect Wind Mechanical

Pressurization

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17

Air Leakage

Importance of Air leakage control

• Air flow carries:

• heat (affects thermal performance),

• rain (increases risk of water leaks)

• water vapor (may result in condensation within

assemblies)

Durability

Characteristics of Climates in

Oregon– Mixed-Marine

• Mean Temperature of Coldest Month between 27°F

and 65°F

• Warmest month mean temperature less than 72°F

• At least 4 months mean temperature over 50°F

• Dry season in summer. Month with heaviest

precipitation in cold season has at least 3x as much

precipitation as the month with the least precipitation in

the rest of year. Cold season is October through

March.

• Heating Degree Days (base 65°F) between 3600 and

5400

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Characteristics of Climates in

Pacific Northwest – Cool-Dry

• Not marine and:

• Annual precipitation (inches) is less than 0.44 x

(annual mean temperature – 19.5)

• HDD (base 65°F) is between 5400 and 7200

• Less humidity

• Greater temperature extremes

• -40 for several days near Burns (1989)

• 117 degrees in Prineville, Pendleton (1998)

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Climate Zones – IECC, ASHRAE

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Climate Zones of OR State –

per 2009 IECC

C 5B

• 4C = Mixed-Marine

• 5B = Cool-Dry 23

CLIMATE ZONES PER 2010

OEESC

• Two Climate Zones are defined in the OEESC,

Section 301.1

• thermal performance requirements vary based on

Climate Zone

• Climate Zone 4C: West of the Cascades

Benton, Clackamas, Clatsop, Columbia, Coos, Curry, Douglas,

Jackson, Josephine, Lane, Lincoln, Linn, Marion, Multnomah, Polk,

Tillamook, Washington, Yamhill

• Climate Zone 5B: East of the Cascades

Baker, Crook, Deschutes, Gilliam, Grant, Harney, Hood River,

Jefferson, Klamath, Lake, Malheur, Morrow, Sherman, Umatilla, Union,

Wallowa, Wasco, Wheeler

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Rain Penetration Failures

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Rain Penetration Failures

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Rain Penetration Control

For control, resist these forces with • Deflection

• Drainage

• Drying

• Durable materials

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Momentum Capillarity can “pull” water “uphill”

Gravity Air pressure

Driving forces:

Rain Penetration Control

Design

Considerations

Deflection

Drainage

Drying

Durability

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Redundancy

is important to

success

Some common approaches

Mass wall

Face-sealed

Concealed barrier

Rainscreen

Rain Penetration Control

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Rain

Water Shedding

Absorbed Water

Rain Penetration Control

Mass Walls

Mass wall: relies on absorption & evaporation

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Rain

Water Shedding

Absorbed Water

Rain Penetration Control

Mass Walls

Mass wall: relies on absorption & evaporation

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Not all mass

walls are

created equal

Rain Penetration Control

Mass Walls

35

Rain Penetration Control

Face Seals

Face seal (Barrier): relies on sealed exterior

36

Rain Penetration Control

Face Seals

Face seal (Barrier): relies on sealed exterior

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Rain Penetration Control

Concealed Barriers

Concealed barrier: relies on multiple layers

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Rain Penetration Control

Concealed Barriers

Concealed barrier: relies on multiple layers

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Rain Penetration Control

Rainscreens

Rainscreen: relies on 2 layers with air space and drainage

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Rain Penetration Control

Rainscreens

Rainscreen: relies on 2 layers with air space and drainage

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Requirements of Rainscreen Walls

• Water shedding surface

(Rainscreen)

• Secondary moisture barrier

(water resistive barrier)

• Drainage path from water

barrier to outside

• Ventilated cavity increases

drying potential

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Water Resistive Barrier

Water Shedding Materials

• Asphalt impregnated paper (building paper)

• Spun-bonded polyolefin or similar “house wraps"

• Some fluid-applied membranes

• Water repellant coatings

Waterproof Materials

• some Fluid-Applied membranes

• some Self-Adhered membranes

• Torch Applied membranes

43

Water Resistive Barrier

May also perform function of:

• Air barrier

• Vapor retarder • but must have insulation installed to exterior!

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Why Control Air Flow?

• Air leakage will carry moisture laden air into the envelope, and interstitial condensation can lead to mold, rot, corrosion

• Higher energy cost

• Poor thermal comfort (winter and summer)

• Uncontrolled indoor environment (humidity, outdoor contaminants)

• Larger forces for rain penetration

• Possible fire/smoke movement

• Increased sound transmission

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Air Leakage Problems

Failure at Roof Parapet

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What Happens Without Envelope Continuity?

47

What Happens Without Envelope Continuity?

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Spray foam insulation for

air, vapor and thermal

continuity

Failure at Soffits and Decks

Air Leakage Problems

49

Air Leakage Problems

Window Interface

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Categorizing Air Leakage

Orifice Flow: air flow through direct opening localized leakage path condensation and water intrusion

Pao Pai

Diffuse Flow: air flow through material large areas condensation over time

Channel Flow: air flow through tortuous path localized leakage path highest condensation potential

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Air Leakage Path and

Condensation

75% 32% 29% 71%

52 Percentage shows relative amount of moisture that can be deposited in a wall cavity based on leakage path.

Controlling Air Flow

What is an Air Barrier System?

The assembly installed to provide a

continuous barrier to the movement of air

across the building envelope

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Air Flow Control

Design Considerations

• Install ‘plane of air tightness’ using low air

permeance materials (low air flow)

• Design system to resist air pressure difference

especially wind loads (structural integrity)

• Maintain low air flow and structural integrity

across joints and junctions (continuity)

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Vapor Diffusion Control

Vapor retarder: the material(s) installed to control the

diffusion of water vapor

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Vapor Diffusion Control

• Low vapor permeance material must be warm

enough to avoid moisture accumulation from

condensation (installed to the interior of thermal

insulation in Pacific Northwest)

• Assembly should be more permeable towards

the low vapor pressure side of the envelope

assembly

• Vapor retarder need not be sealed to control

vapor diffusion (if there is an air barrier and if

conditioned air cannot “ bypass” the vapor

retarder to circulates behind it)

Design Considerations

interior

exterior

building envelope

56

Heat Flow

57

Heat Flow Problems

58

Thermal Barrier (Insulation)

• The element that represents the majority of

resistance to conductive heat flow in an assembly

• Must be continuous but does not need to be sealed

• Best placed to the exterior of the structure to

minimize thermal bridging

• Ideally majority placed outboard of air barrier

• Majority must be placed outboard of vapor barrier

(except in cooling climates)

• Placed inboard of weather barrier unless water

insusceptible

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Heat Flow Control

Design Considerations

• Design for continuity of coverage

• Minimize thermal bridges

• Avoid air flow through/around insulation

• Keep low permeance material warm enough to

avoid condensation

Heat Flow Control – Thermal

Mass

Image from www.concretethinker.com

61

Below Grade Design

Considerations

• Control of groundwater

• Control of radon gas

• Control of potential soil contaminates

• Continuity of barriers at transition from

below grade to above grade assemblies

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Architectural expressions impacting

building envelope performance

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Architectural expressions impacting

building envelope performance

Source: Glass and Metal Curtain Wall Best Practice Guide, Canada Mortgage and Housing Corporation.

Vertical mullion

Air space Rigid insulation

Flexible air barrier membrane

Snap cap

Pressure plate

Sealed glazing unit

Anti-rotation spacer Galvanized metal

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Architectural expressions impacting

building envelope performance

Slab projections/decks,

etc.

• Good for rainwater

penetration control: protects

walls and windows from

wind driven rain

• Bad for thermal control:

large thermal bridges

through insulation

65

Architectural expressions impacting

building envelope performance

Projecting decorative

trim

• Can collect water,

increasing risk of water

penetration and

deterioration of

claddings/trim.

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